x86/mm: Prevent non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border

In case of 5-level paging, the kernel does not place any mapping above
47-bit, unless userspace explicitly asks for it.

Userspace can request an allocation from the full address space by
specifying the mmap address hint above 47-bit.

Nicholas noticed that the current implementation violates this interface:

  If user space requests a mapping at the end of the 47-bit address space
  with a length which causes the mapping to cross the 47-bit border
  (DEFAULT_MAP_WINDOW), then the vma is partially in the address space
  below and above.

Sanity check the mmap address hint so that start and end of the resulting
vma are on the same side of the 47-bit border. If that's not the case fall
back to the code path which ignores the address hint and allocate from the
regular address space below 47-bit.

To make the checks consistent, mask out the address hints lower bits
(either PAGE_MASK or huge_page_mask()) instead of using ALIGN() which can
push them up to the next boundary.

[ tglx: Moved the address check to a function and massaged comment and
  	changelog ]

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20171115143607.81541-1-kirill.shutemov@linux.intel.com

1 files changed, 7 insertions, 3 deletions

diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
index a63fe77b3217..676774b9bb8d 100644
--- a/arch/x86/kernel/sys_x86_64.c
+++ b/arch/x86/kernel/sys_x86_64.c
@@ -188,6 +188,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
 	if (len > TASK_SIZE)
 		return -ENOMEM;
 
+	/* No address checking. See comment at mmap_address_hint_valid() */
 	if (flags & MAP_FIXED)
 		return addr;
 
@@ -197,12 +198,15 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
 
 	/* requesting a specific address */
 	if (addr) {
-		addr = PAGE_ALIGN(addr);
+		addr &= PAGE_MASK;
+		if (!mmap_address_hint_valid(addr, len))
+			goto get_unmapped_area;
+
 		vma = find_vma(mm, addr);
-		if (TASK_SIZE - len >= addr &&
-				(!vma || addr + len <= vm_start_gap(vma)))
+		if (!vma || addr + len <= vm_start_gap(vma))
 			return addr;
 	}
+get_unmapped_area:
 
 	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
 	info.length = len;